My Lady and the other worthy posters have covered most of the titles that I’ve thought of, though I must add
The Dark is Rising by Susan Cooper
Hero and the Crown by Robin McKinley
I also wanted to share a few quotes attributed to C.S. Lewis that I try to keep in mind:
No book is really worth reading at age of ten which is not equally (and often far more) worth reading at the age of fifty–except, of course, books of information. The only imaginative works we ought to grow out of are those which it would have been better not to have read at all.
…a children’s story which is only enjoyed by children is a bad children’s story.
When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifth, I read them openly. When I became a man, I put away childish things, including the fear of childishness and the desire to be very grown up.
Those of us who are blamed when old for reading childish books were blamed when children for reading books too old for us. No reader worth his salt trots along in obedience to a time table.